PROJECT SUMMARY ! Dengue viruses are mosquito-borne flaviviruses of immense public health impact that cause a spectrum of disease in humans ranging from mild to fatal. Progression to severe dengue disease is promoted by the presence of non-neutralizing anti-dengue IgGs that modulate virus and cytokine production in Fc receptor- bearing cells. We have shown that progression to severe dengue disease is promoted by the presence of anti- dengue antibodies with abundant afucosylated Fc glycoforms, a modification that enhances affinity of the Fc for a specific activating Fc receptor, Fc?RIIIa. Thus, our data point to a role for Fc?RIIIa in the pathogenesis of dengue disease. In this proposal we will study samples from Phase III trials of a live, attenuated tetravalent dengue virus vaccine, CYD-TDV (Dengvaxia, Sanofi Pasteur), to define mechanism involved in human immunity to dengue viruses. Recent analyses of data from the Phase III trials of CYD-TDV showed that risk for disease was increased in some study cohorts by vaccination. This finding highlights the importance of understanding how antibody responses to dengue vaccination are regulated and molecular mechanisms by which antibodies can enhance dengue infections. Aims in this proposal will: a) define regulators of Fc fucosylation on antibodies elicited by CYD-TDV vaccination or by natural dengue infection in humans; b) define associations between post- vaccination/pre-infection anti-dengue antibody repertoires and susceptibility to dengue disease during a 5-year follow-up period after vaccination; c) define mechanisms by which Fc?RIIIa impacts dengue infections and disease pathogenesis. Collectively, these aims will advance our fundamental understanding of mechanisms regulating human immunity to dengue viruses and guide the design of safe, effective dengue virus vaccines.